The present invention relates generally to the continuous casting of steel and to the special ceramic components which are employed therein. More particularly, the invention concerns a ceramic pouring tube, commonly referred to as a ladle shroud, which permits the transfer of molten metal from a ladle to a casting tundish located beneath the ladle. Molten metal is then directed from the tundish into a continuous casting mold or molds positioned beneath the tundish, all in a well-known manner. In a typical continuous steel casting operation, a ceramic collector nozzle is fitted beneath a bottom orifice of the refractory-lined ladle. Control of molten metal flow from the ladle through the collector nozzle is accomplished by either a vertically movable stoppers rod which incrementally opens and closes the opening in the upper end of the collector nozzle, or it is controlled by way of a conventional slide gate plate valve arrangement in which the collector nozzle is mounted on a bottom plate thereof. Relative movement of the slide gate plates opens and closes the metal path to the collector nozzle. The ladle shroud to which the present invention pertains is snugly fitted beneath the collector nozzle to permit the pouring (teeming) of molten metal from the ladle to the tundish located below.
Ladle shrouds are commonly used in the continuous casting of steel to prevent oxidation of the stream of molten steel, as the metal is teemed from the ladle to the tundish, and to protect workers in the casting area from being burned by splashing metal. One of the major problems heretofore encountered in the use of such ladle shrouds is the ability to obtain a tight seal between the collector nozzle and the top of the ladle shroud in the joint area where these components are fitted together. A poor seal at this joint interface results in air infiltration causing objectionable oxidation of the molten steel. It is often very difficult to consistently guarantee the quality of this joint seal due to steel splashing on the collector nozzle or due to handling damage inflicted at the top of the ladle shroud when it is disconnected from a spent ladle and reconnected to a full ladle.
Heretofore, it has been proposed to inject an inert gas around the top of a ladle shroud to provide a gas seal in the event a poor mechanical joint with the collector nozzle is present. The inert gas is nonreactive with the molten steel and when a poor seal is present, the inert gas, such as argon, floods the seal area of the joint preventing the infiltration of air and subsequent oxidation of the molten steel.
Several ladle shroud designs have been heretofore proposed to facilitate the injection of argon around the collector nozzle for sealing purposes. One such design is disclosed in U.S. Pat. No. 4,519,438. In this design, the argon is injected in finger-like grooves which are pressed into the top of the ladle shroud. This design, while partially successful, has some drawbacks. Often the grooves fill up with steel during the casting, making the subsequent injection of inert gas troublesome. It is also often difficult to obtain uniform gas distribution around the seal in that the inert gas has a tendency to assume a higher pressure in the region directly behind the gas inlet connection. As a result, higher flow rates of inert gas injection are necessary to obtain improved gas distribution.
A further attempted solution incorporates a ring of a porous ceramic material in place of the grooves for the injection of argon. Normally the ladle shroud body is composed of an alumina graphite material due to its high thermal shock resistance and ability to withstand attack by molten steel and slag. In this prior art ladle shrod design, the porous ring material does not employ the alumina graphite composition of the body, but rather is a 100% oxide composition, usually consisting of alumina-silicates. This prior porous ring solved one problem present in the groove-type design in that there is no steel infiltration into the porous ring material, and hence a uniform gas flow rate can be obtained due to the back pressure provided thereby. However, other problems with this prior porous ring design are also encountered. The porous ring is usually inserted into the ladle shroud body after both pieces (ring and shroud body) have been completely finished. This requires a difficult procedure of cementing the porous ring into place, and then encasing the top of the composite ladle shroud in a special steel can. The can serves the purpose of holding the porous ring in place, and insures that a gas-tight seal is obtained around the top of the shroud. If the can is accidentally penetrated or expands, the inert gas will leak either through the hole, or around the top of the shroud. In either case, this greatly reduces the effectiveness of the seal since the gas is not being placed where it is most needed. The probability of having this happen during a casting sequence is quite high.
It has also been proposed to form a preformed ring of porous ceramic material and then co-press the porous ring with the shroud body wherein the porous ring has an inner surface co-extensive with the bore of the shroud and an upper surface co-extensive with the upper surface of the shroud. The co-pressed ring and shroud body with a preformed gas channel are then fired. The resultant fired piece, while offering improved bonding between the porous ring and the body, still requires the use of a steel can with a steel cap portion to prevent inert gas leakage from the exposed top surface area of the porous ring.
The present invention solves these and other shortcomings found in the prior art by providing a ladle shroud having a co-pressed, gas permeable ring which is substantially more effective than commonly used argon shrouds in protecting molten metal from the harmful effects of air infiltration. Unwanted oxidation and nitrogen pickup in the teemed steel is, thus, significantly reduced through the use of the ladle shroud of the present invention. Still further, the co-pressed ladle shroud and gas permeable ring of the present invention is less expensive to manufacture than known gas permeable ladle shrouds due, in part, to the fact that it requires no separate cementing operation and no metal can component for encasing the upper ring surface for sealing against gas leakage.
In addition, the integral porous ring portion and the dense ladle shroud of the invention both are of essentially the same ceramic composition so as to provide uniform thermal expansion and contraction properties therebetween whereby thermal cracking problems are minimized. The invention further provides an integral porous ring having an improved bond with the body which requires no metal cap or cement to hold it in place within the ladle shroud. Still further, improved gas distribution is obtained in the integral porous ring of the present invention through controlled refractory grain sizing in the initial mix which yields a uniform mean pore diameter size after firing.